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TEMA换热器设计 Thermal design of shell-and-tubeheat exchangers (STHEs) isdone by sophisticated computersoftware. However, a good un- derstanding of the underlying principles of exchanger design is needed to use this software effectively. This article explains the basics of...

TEMA换热器设计
Thermal design of shell-and-tubeheat exchangers (STHEs) isdone by sophisticated computersoftware. However, a good un- derstanding of the underlying principles of exchanger design is needed to use this software effectively. This article explains the basics of ex- changer thermal design, covering such topics as: STHE components; classifica- tion of STHEs according to construction and according to service; data needed for thermal design; tubeside design; shellside design, including tube layout, baffling, and shellside pressure drop; and mean temperature difference. The basic equa- tions for tubeside and shellside heat transfer and pressure drop are well- known; here we focus on the application of these correlations for the optimum de- sign of heat exchangers. A followup arti- cle on advanced topics in shell-and-tube heat exchanger design, such as allocation of shellside and tubeside fluids, use of multiple shells, overdesign, and fouling, is scheduled to appear in the next issue. Components of STHEs It is essential for the designer to have a good working knowledge of the mechani- cal features of STHEs and how they in- fluence thermal design. The principal components of an STHE are: • shell; • shell cover; • tubes; • channel; • channel cover; • tubesheet; • baffles; and • nozzles. Other components include tie-rods and spacers, pass partition plates, impinge- ment plate, longitudinal baffle, sealing strips, supports, and foundation. The Standards of the Tubular Ex- changer Manufacturers Association (TEMA) (1) describe these various com- ponents in detail. An STHE is divided into three parts: the front head, the shell, and the rear head. Figure 1 illustrates the TEMA nomenclature for the various construction possibilities. Exchangers are described by the letter codes for the three sections — for example, a BFL exchanger has a bon- net cover, a two-pass shell with a longitu- dinal baffle, and a fixed-tubesheet rear head. Classification based on construction Fixed tubesheet. A fixed-tubesheet heat exchanger (Figure 2) has straight tubes that are secured at both ends to tubesheets welded to the shell. The con- struction may have removable channel covers (e.g., AEL), bonnet-type channel covers (e.g., BEM), or integral tubesheets (e.g., NEN). The principal advantage of the fixed- tubesheet construction is its low cost be- cause of its simple construction. In fact, the fixed tubesheet is the least expensive construction type, as long as no expan- sion joint is required. Other advantages are that the tubes can be cleaned mechanically after removal of SHELL-AND-TUBE HEAT EXCHANGERS CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998 ©Copyright 1997 American Institute of Chemical Engineers. All rights reserved. Copying and downloading permitted with restrictions. Effectively Design Shell-and-Tube Heat Exchangers Rajiv Mukherjee, Engineers India Ltd. To make the most of exchanger design software, one needs to understand STHE classification, exchanger components, tube layout, baffling, pressure drop, and mean temperature difference. heqingyang 线条 CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998 SHELL-AND-TUBE HEAT EXCHANGERS n Figure 1. TEMA designations for shell-and-tube heat exchangers. E F G H J K X One-Pass Shell Two-Pass Shell with Longitudinal Baffle Split Flow Double Split Flow Divided Flow Cross Flow Kettle-Type Reboiler A B Removable Channel and Cover C N Bonnet (Integral Cover) Integral With Tubesheet Removable Cover D Special High-Pressure Closures T U W U-Tube Bundle Pull-Through Floating Head Floating Head with Backing Device S P N Outside Packed Floating Head Fixed Tube Sheet Like "C" Stationary Head Fixed Tube Sheet Like "B" Stationary Head Externally Sealed Floating Tubesheet Fixed Tube Sheet Like "A" Stationary Head Stationary Head Types Shell Types Rear Head Types M L Channel Integral With Tubesheet and Removable Cover heqingyang 矩形 heqingyang 矩形 heqingyang 矩形 heqingyang 矩形 the channel cover or bonnet, and that leakage of the shellside fluid is mini- mized since there are no flanged joints. A disadvantage of this design is that since the bundle is fixed to the shell and cannot be removed, the out- sides of the tubes cannot be cleaned mechanically. Thus, its application is limited to clean services on the shell- side. However, if a satisfactory chem- ical cleaning program can be em- ployed, fixed-tubesheet construction may be selected for fouling services on the shellside. In the event of a large differential temperature between the tubes and the shell, the tubesheets will be un- able to absorb the differential stress, thereby making it necessary to incor- porate an expansion joint. This takes away the advantage of low cost to a significant extent. U-tube. As the name implies, the tubes of a U-tube heat exchanger (Figure 3) are bent in the shape of a U. There is only one tubesheet in a U- tube heat exchanger. However, the lower cost for the single tubesheet is offset by the additional costs incurred for the bending of the tubes and the somewhat larger shell diameter (due to the minimum U-bend radius), mak- ing the cost of a U-tube heat ex- changer comparable to that of a fixed- tubesheet exchanger. The advantage of a U-tube heat exchanger is that because one end is free, the bundle can expand or con- tract in response to stress differen- tials. In addition, the outsides of the tubes can be cleaned, as the tube bun- dle can be removed. The disadvantage of the U-tube construction is that the insides of the tubes cannot be cleaned effectively, since the U-bends would require flex- ible-end drill shafts for cleaning. Thus, U-tube heat exchangers should not be used for services with a dirty fluid inside tubes. Floating head. The floating-head heat exchanger is the most versatile type of STHE, and also the costliest. In this design, one tubesheet is fixed relative to the shell, and the other is free to “float” within the shell. This permits free expansion of the tube bundle, as well as cleaning of both the insides and outsides of the tubes. Thus, floating-head SHTEs can be used for services where both the shellside and the tubeside fluids are dirty — making this the standard con- struction type used in dirty services, such as in petroleum refineries. There are various types of float- ing-head construction. The two most common are the pull-through with backing device (TEMA S) and pull- through (TEMA T) designs. The TEMA S design (Figure 4) is the most common configuration in the chemical process industries (CPI). The floating-head cover is secured against the floating tubesheet by bolt- ing it to an ingenious split backing ring. This floating-head closure is lo- cated beyond the end of the shell and contained by a shell cover of a larger diameter. To dismantle the heat ex- changer, the shell cover is removed first, then the split backing ring, and then the floating-head cover, after which the tube bundle can be re- moved from the stationary end. In the TEMA T construction (Fig- ure 5), the entire tube bundle, includ- ing the floating-head assembly, can be removed from the stationary end, since the shell diameter is larger than the floating-head flange. The floating- head cover is bolted directly to the floating tubesheet so that a split back- ing ring is not required. The advantage of this construction is that the tube bundle may be re- moved from the shell without remov- ing either the shell or the floating- head cover, thus reducing mainte- nance time. This design is particular- ly suited to kettle reboilers having a dirty heating medium where U-tubes cannot be employed. Due to the en- larged shell, this construction has the highest cost of all exchanger types. FEBRUARY 1998 • CHEMICAL ENGINEERING PROGRESS Support Bracket Stationary Tubesheet Stationary Tubesheet Bonnet (Stationary Head) Bonnet (Stationary Head) Baffles Tie Rods and Spacers n Figure 2. Fixed-tubesheet heat exchanger. Tubeplate Shell Tubes BafflesHeader n Figure 3. U-tube heat exchanger. heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 线条 heqingyang 矩形 heqingyang 矩形 heqingyang 矩形 heqingyang 矩形 heqingyang 矩形 There are also two types of packed floating-head construction — outside- packed stuffing-box (TEMA P) and outside-packed lantern ring (TEMA W) (see Figure 1). However, since they are prone to leakage, their use is limited to services with shellside flu- ids that are nonhazardous and non- toxic and that have moderate pres- sures and temperatures (40 kg/cm2 and 300°C). Classification based on service Basically, a service may be single- phase (such as the cooling or heating of a liquid or gas) or two-phase (such as condensing or vaporizing). Since there are two sides to an STHE, this can lead to several combinations of services. Broadly, services can be classified as follows: • single-phase (both shellside and tubeside); • condensing (one side condens- ing and the other single-phase); • vaporizing (one side vaporizing and the other side single-phase); and • condensing/vaporizing (one side condensing and the other side vaporizing). The following nomenclature is usually used: Heat exchanger: both sides single- phase and process streams (that is, not a utility). Cooler: one stream a process fluid and the other cooling water or air. Heater: one stream a process fluid and the other a hot utility, such as steam or hot oil. Condenser: one stream a condens- ing vapor and the other cooling water or air. Chiller: one stream a process fluid being condensed at sub-atmo- spheric temperatures and the other a boiling refrigerant or process stream. Reboiler: one stream a bottoms stream from a distillation column and the other a hot utility (steam or hot oil) or a process stream. This article will focus specifically on single-phase applications. Design data Before discussing actual thermal design, let us look at the data that must be furnished by the process li- censor before design can begin: 1. flow rates of both streams. 2. inlet and outlet temperatures of both streams. 3. operating pressure of both streams. This is required for gases, especially if the gas density is not furnished; it is not really necessary for liquids, as their properties do not vary with pressure. 4. allowable pressure drop for both streams. This is a very important parameter for heat exchanger design. Generally, for liquids, a value of 0.5–0.7 kg/cm2 is permitted per shell. A higher pressure drop is usually war- ranted for viscous liquids, especially in the tubeside. For gases, the allowed value is generally 0.05–0.2 kg/cm2, with 0.1 kg/cm2 being typical. 5. fouling resistance for both streams. If this is not furnished, the designer should adopt values speci- fied in the TEMA standards or based on past experience. 6. physical properties of both streams. These include viscosity, thermal conductivity, density, and specific heat, preferably at both inlet and outlet temperatures. Viscosity data must be supplied at inlet and outlet temperatures, especially for liquids, since the variation with tem- perature may be considerable and is irregular (neither linear nor log-log). 7. heat duty. The duty specified should be consistent for both the shellside and the tubeside. 8. type of heat exchanger. If not furnished, the designer can choose this based upon the characteristics of the various types of construction de- scribed earlier. In fact, the designer is normally in a better position than the process engineer to do this. 9. line sizes. It is desirable to match nozzle sizes with line sizes to avoid expanders or reducers. Howev- er, sizing criteria for nozzles are usu- ally more stringent than for lines, es- pecially for the shellside inlet. Conse- quently, nozzle sizes must sometimes be one size (or even more in excep- tional circumstances) larger than the corresponding line sizes, especially for small lines. 10. preferred tube size. Tube size is designated as O.D. · thickness · length. Some plant owners have a preferred O.D. · thickness (usually based upon inventory considerations), and the available plot area will deter- mine the maximum tube length. Many plant owners prefer to stan- dardize all three dimensions, again based upon inventory considerations. 11. maximum shell diameter. This is based upon tube-bundle removal re- quirements and is limited by crane ca- pacities. Such limitations apply only to exchangers with removable tube bun- dles, namely U-tube and floating-head. For fixed-tubesheet exchangers, the only limitation is the manufacturer’s fabrication capability and the avail- ability of components such as dished ends and flanges. Thus, floating-head heat exchangers are often limited to a shell I.D. of 1.4–1.5 m and a tube length of 6 m or 9 m, whereas fixed- tubesheet heat exchangers can have shells as large as 3 m and tubes lengths up to 12 m or more. 12. materials of construction. If the tubes and shell are made of iden- tical materials, all components should be of this material. Thus, only the shell and tube materials of construc- tion need to be specified. However, if the shell and tubes are of different metallurgy, the materials of all princi- pal components should be specified to avoid any ambiguity. The principal components are shell (and shell cover), tubes, channel (and channel cover), tubesheets, and baffles. Tubesheets may be lined or clad. 13. special considerations. These include cycling, upset conditions, al- ternative operating scenarios, and whether operation is continuous or intermittent. Tubeside design Tubeside calculations are quite straightforward, since tubeside flow CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998 SHELL-AND-TUBE HEAT EXCHANGERS heqingyang 线条 heqingyang 线条 represents a simple case of flow through a circular conduit. Heat-trans- fer coefficient and pressure drop both vary with tubeside velocity, the latter more strongly so. A good design will make the best use of the allowable pressure drop, as this will yield the highest heat-transfer coefficient. If all the tubeside fluid were to flow through all the tubes (one tube pass), it would lead to a certain veloc- ity. Usually, this velocity is unaccept- ably low and therefore has to be in- creased. By incorporating pass parti- tion plates (with appropriate gasket- ing) in the channels, the tubeside fluid is made to flow several times through a fraction of the total number of tubes. Thus, in a heat exchanger with 200 tubes and two passes, the fluid flows through 100 tubes at a time, and the velocity will be twice what it would be if there were only one pass. The number of tube passes is usually one, two, four, six, eight, and so on. Heat-transfer coefficient The tubeside heat-transfer coeffi- cient is a function of the Reynolds number, the Prandtl number, and the tube diameter. These can be bro- ken down into the following funda- mental parameters: physical properties (namely viscosity, ther- mal conductivity, and specific heat); tube diameter; and, very important- ly, mass velocity. The variation in liquid viscosity is quite considerable; so, this physical property has the most dramatic effect on heat-transfer coefficient. The fundamental equation for tur- bulent heat-transfer inside tubes is: Nu = 0.027 (Re)0.8 (Pr)0.33 (1a) or (hD/k) = 0.027 (DG/m )0.8 (cm /k)0.33 (1b) Rearranging: h = 0.027(DG/m )0.8(cm /k)0.33(k/D) (1c) Viscosity influences the heat-trans- fer coefficient in two opposing ways — as a parameter of the Reynolds number, and as a parameter of Prandtl number. Thus, from Eq. 1c: h a (m )0.33–0.8 (2a) h a (m )–0.47 (2b) In other words, the heat-transfer coefficient is inversely proportional to viscosity to the 0.47 power. Simi- larly, the heat-transfer coefficient is directly proportional to thermal con- ductivity to the 0.67 power. These two facts lead to some inter- esting generalities about heat transfer. A high thermal conductivity promotes a high heat-transfer coefficient. Thus, cooling water (thermal conductivity of around 0.55 kcal/h•m•°C) has an extremely high heat-transfer coeffi- cient of typically 6,000 kcal/h•m2•°C, followed by hydrocarbon liquids (thermal conductivity between 0.08 and 0.12 kcal/h•m•°C) at 250–1,300 kcal/h•m2•°C, and then hydrocarbon gases (thermal conductivity between 0.02 and 0.03 kcal/h•m•°C) at 50–500 kcal/h•m2•°C. Hydrogen is an unusual gas, be- cause it has an exceptionally high thermal conductivity (greater than that of hydrocarbon liquids). Thus, its heat-transfer coefficient is to- ward the upper limit of the range for hydrocarbon liquids. The range of heat-transfer coeffi- cients for hydrocarbon liquids is FEBRUARY 1998 • CHEMICAL ENGINEERING PROGRESS Stationary-Head Channel Stationary Tubesheet Shell Support Saddles Floating Tubesheet Floating-Head Cover Shell Cover Tie Rods and Spacers Pass Partition Baffles n Figure 4. Pull-through floating-head exchanger with backing device (TEMA S). Shell Weir Support SaddleBaffles Support Saddle Floating Tubesheet Floating-Head Cover Shell CoverStationary-Head Channel Tie Rods and Spacers Pass Partition n Figure 5. Pull-through floating-head exchanger (TEMA T). rather large due to the large variation in their viscosity, from less than 0.1 cP for ethylene and propylene to more than 1,000 cP or more for bitumen. The large variation in the heat-transfer coefficients of hydrocarbon gases is attributable to the large variation in operating pressure. As operating pres- sure rises, gas density increases. Pres- sure drop is directly proportional to the square of mass velocity and in- versely proportional to density. There- fore, for the same pressure drop, a higher mass velocity can be main- tained when the density is higher. This larger mass velocity translates into a higher heat-transfer coefficient. Pressure drop Mass velocity strongly influences the heat-transfer coefficient. For tur- bulent flow, the tubeside heat-transfer coefficient varies to the 0.8 power of tubeside mass velocity, whereas tube- side pressure drop varies to the square of mass velocity. Thus, with increas- ing mass velocity, pressure drop in- creases more rapidly than does the heat-transfer coefficient. Consequent- ly, there will be an optimum mass ve- locity above which it will be wasteful to increase mass velocity further. Furthermore, very high velocities lead to erosion. However, the pres- sure drop limitation usually becomes controlling long before erosive veloc- ities are attained. The minimum rec- ommended liquid velocity inside tubes is 1.0 m/s, while the maximum is 2.5–3.0 m/s. Pressure drop is proportional to the square of velocity and the total length of travel. Thus, when the num- ber of tube passes is increased for a given number of tubes and a given tubeside flow rate, the pressure drop rises to the cube of this increase. In actual practice, the rise is somewhat less because of lower friction factors at higher Reynolds numbers, so the exponent should be approximately 2.8 instead of 3. Tubeside pressure drop rises steeply with an increase in the number of tube passes. Consequently, it often happens that for a given number of tubes and two passes, the pressure drop is much lower than the allowable value, but with four passes it exceeds the allow- able pressure drop. If in such circum- stances a standard tube has to be em- ployed, the designer may be forced to accept a rather low velocity. However, if the tube diameter and length may be varied, the allowable pressure drop can be better utilized and a higher tubeside velocity realized. The following tube diameters are usually used in the CPI: w, 1, e, 5, 1, 14, and 11 in. Of these, 5 in. and
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